In training, Morihiro Saito would say that both uke and nage should feel like they are moving through molasses. There are a couple of reasons to do this. One is it builds up your muscular strength and postural muscles (internal?) and you will know when your body is not in alignment because it will be very hard to move uke. Also you can feel when you have uke's balance because they will not be able to give much resistance. Basically it is not that good to go to both extremes, like being too cooperative or shutting someone down.